The Blue Planetary Nebula is a planetary nebula located in the southern constellation Centaurus. Popularly known as the Southerner, it is the brightest planetary nebula in the far southern sky. It lies approximately 4,900 light-years away and has an apparent magnitude of 8.5. The nebula is catalogued as NGC 3918 in the New General Catalogue.
The Blue Planetary Nebula was discovered by the English astronomer Sir John Herschel in March 1834. With an apparent size between 8 and 10 arcseconds, it makes a good target for small telescopes. In long-exposure images, the nebula’s oval diameter appears considerably larger, about 19 or 20 arcseconds across.
NGC 3918 was named the Blue Planetary Nebula because of its rich blue colour and appearance, which is reminiscent of the images of the planet Neptune taken by the space probe Voyager 2 in 1989. In 10-inch telescopes, the nebula appears as a small bluish disk at low power.
The nebula was formed when a red giant star reached the end of its life and expelled its outer layers into space. The ejected material is illuminated by the intense ultraviolet light of the hot central stellar remnant which will eventually cool and fade. Planetary nebulae like NGC 3918 represent the final stages in the evolutionary cycle for most stars. The same fate awaits our own Sun in about 4-5 billion years.
The Blue Planetary Nebula expands at a rate of about 24 kilometres per second. It is approaching us at about 17 km/s.
The central star, catalogued as HD 102854, has an apparent magnitude of 14.6 and is well below the limit of unaided eye visibility. Even though it has a luminosity 6,000 times that of the Sun, the star is obscured by the light of the surrounding nebula.
The Blue Planetary Nebula was first imaged by NASA and ESA’s Hubble Space Telescope (HST) in 1998. The image revealed the shells of material expelled from the star. The shells form a spherical outer envelope and an elongated inner balloon, which is shaped by the fast stellar wind from the central star. The shells form an eye-like shape and appear like concentric rings around the hot progenitor star. They extend far beyond the nebula and will eventually dissipate into the surrounding space. The lifespan of planetary nebulae is only a few tens of thousands of years.
The Hubble telescope captured the Southerner again in 2010. The composite image of visible and near-infrared shots was taken with the Wide Field and Planetary Camera 2 and provided a better look into the details of the nebula.
The Blue Planetary Nebula is relatively easy to find because it lies near the Southern Cross, one of the most recognizable asterisms in the far southern sky. The nebula appears very close to the imaginary line drawn through the Southern Cross, from Mimosa (Beta Crucis) to Imai (Delta Crucis) and extended by almost the same distance as that between the two stars. The nebula appears within an elongated triangle of fainter stars, about 2.5 degrees northwest of Imai.
The best time of the year to observe the Blue Planetary Nebula and other deep sky objects in Centaurus is during the month of May, when the constellation is high above the horizon in the evening.
The nebula never rises high above the horizon for observers in northern latitudes. It is invisible from locations north of the latitude 32° N.
Blue Planetary Nebula – NGC 3918
|11h 50m 17.7709504944s
|−57° 10′ 57.017302428″
|0.26841834 x 0.26777998 arcminutes
|4,900 light-years (1.5 kiloparsecs)
|Names and designations
|Blue Planetary Nebula, the Southerner, NGC 3918, Hen 2-74, PK 294+4.1, ESO 170-1, ESO 170-13, Sa2-81, PN G294.6+04.7, AM 1147-565, ARO 514, VV 61, WRAY 16-101, TYC 8639-2315-1, HD 102854, Gaia DR2 5342260406474812416, Gaia DR3 5342260406474812416